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IMPROVED RESEALING PROCEDURES FOR THE W

SECOND DESEAL/RESEAL PROGRAM IN RAAFFl 11 AIRCRAFT FUEL TANKS

S. BAUrEN, R.h.E. HUANG AND L.V. WAKE

MRL-TR-93-64

JANUARY 1994

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MAR 18 1994

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Improved Resealing Procedures for theSecond Deseal/Reseal Program in RAAF

Flll Aircraft Fuel Tanks

S. Batten, R.H.E. Huang and L.V. Wake

MRL Technical ReportMRL-TR-93-64

Abstract

A high temperature-resistant polyester sealant was originally employed to sealfayingsurface grooves in F1Il fuselage fuel tanks as well as structural voids and aerodynamicsurfaces. Chemical hydrolysis of this material was detected in March 1974 and led to itsreplacement by a polysulfide sealant in the late 70's and early 80's. Where possible, thepolyester was removed, however much of it remained inaccessibly located between theoverlapping panels. It was found that the continued -drolysis of this residual polyestergenerated sufficient pressure to penetrate the polysulfde sealant resulting in fuel leaks aswell as appearing externally in trails along the skin of the aircraft. To protect thepolysu fiefrom penetration, a mechanical barrier uras applied in the form of a layer ofepoxy polyamide between the polysulfide and the polyester. This system was successfulforsome years, however the occurrence offuel leaks had become unacceptable by 1987. All NF 11 aircraft suffered numerous fuel leaks over the period 1988-91 including a number of U -safety-of-flight (SOF) incidents. A detailed investigation was therefore requested to Pident6fy the cause(s) of sealant foilures. This investigation found that the sealant and the 0ounderlying epoxy barrier coating could be manually peeled frvm the painted tank surface 0when prepared by the recommended methods and that two procedures were responsible forthe poor adhesion. One of the procedures involved application of the barrier coating over Mincompletely cured priming paint resulting in solvent attack of the barrier. The second 0')procedure, involving the use of a so-called "titanate adhesion promoter', signifcantlyreduced peel strength of the barrier coating to the paint. Modification of these two stepsresulted in adhesive bond strengths greater than the cohesive strength of the polysulfide

sealant. These changes were adopted for the second deseal/reseal program of the F1ill0jaircraft fleet. Following teething problems with the first aircraft, the procedures wereimproved and the Fll fleet remains leak-free after two years.

DEPARTMENT OF DEFENCE

DSTO MATERIALS RESEARCH LABORATORY

si a 183'oO 4 -

Accesion ForNTIS CRAM&uliWbDTIO TAB DSTO Materia Resard Laboratryunannounced GwCrdte Avenue, MarbyrnonJ-istificatiofl. Vktaori,3032 Austraia

By -- --- ... Tele~mw (03) 246 8 111Di-t. i-ti-ior Fax: (03) 246 899k~aiý,iiiiy CdesC Comnhwnueth ofAustralia 9 9 3Avai~bitY CdesAR No. 008-616

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APPROVED FOR PUBLIC RELEASE

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Authors

S. Batten

Stuart Batten undertook work on aspects of the presentreport as a vacation scholar during the periodDecember 1990 - January 1991. Stuart Batten, who was a1st Class Honours Student at the time of this work iscurrently undertaking a PhD at the University ofMelbourne majoring in inorganic chemistry.

* R.H.E. Huang

Robert Huang holds a BSc degree from the University ofAdelaide, South Australia. After two and a half years ofexperience in Rubber Technology with the Olympic Tyreand Rubber Company, Melbourne, he joined the rubbersection of MRL in November 1970. Since 1975 he hasbeen involved in R & D work on sealants as an SPOC.

L.V. Wake

Lindsay Vernon Wake was awarded a Department ofSupply scholarship to complete a PhD at the University ofNSW in 1969. He joined MRL in 1972 and was brieflyassociated with the Textiler Group and later the MarineEnvironment Group of MRL. Since 1981, Dr Wake hasbeen located in Paint Technology where lie has formed aclose working association with the RAAF. He is currentlyinvestigating the synthesis and performance of fireretardant paints, high temperature resistant coatings, NIRcamouflage paints and other coating materials of Defenceinterest.

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Contents

1. INTRODUCTION 7

2. SEALANT FAILURES IN RAAF Fill AIRCRAFT 9

3. EXPERIMENTAL 133.1 Materials 13

3.1.1 Paint 133.1.2 Barrier Coatings 133.1.3 Sealants 143.1.4 AluminiumPanels 14

3.1.5 Adhesion Promoter 143.2 Examinations 14

3.2.1 Modi.ied Peel Test 143.2.2 Modified Lap Shear Test 153.2.3 Fuel Immersion of Barrier Coatings 15

4. RESULTS 154.1 Techniques for Determining Flll Sealant System Performance 154.2 Effect of Sealant Procedures on Adhesion and Cohesion of Sealant

System 164.2.1 Effect of Paint Cure on Peel and Lap Shear Strength 164.2.2 Effect of Adhesion Promoter on Adhesion 234.2.3 Effect of Barrier Coating Composition on Adhesion 234.2.4 Effect of Fuel Immersion Times on Swelling and Adhesion 244.2.5 Effect of Aging of the Barrier Coating 25

5. DISCUSSION 26

6. OPERATION OF THE SECOND DESEAL/RESEAL PROGRAM 29

7. CONCLUSIONS 31

8. REFERENCES 32

i ..

Improved Resealing Procedures for theSecond DesealiReseal Program in RAAF

Flll Aircraft Fuel Tanks

1. IntroductionThe fuel capacity of FI ll aircraft, as in other modern aircraft, is achieved throughthe concept of integral fuel tanks in which use is made of internal spaces in thefuselage, wings and vertical fins (stabilisers). This concept requires the sealing ofcavities, rivets and seams with flexible sealants which are required to resistturbulence, absorb shock differentials, provide corrosion protection, tolerate allweather conditions and retain fuel. The first sealants meeting these requirementswere based on polysulfide materials cured by calcium dichromate. Later,manganese dioxide cured polysulfide systems were introduced with specificgravity modifications.

Two types of sealants were initially applied to the integral fuel tanks of F111aircraft, these being (i) a polysulfide sealant applied as an internal fillet and onseams and fasteners within tanks and (ii) a polyester sealant applied betweenfaying, i.e. overlapping, surfaces and externally in grooves and voids as anaerodynamic filler (Figure 1).

The polysulfide sealant (i) was applied in a two-stage process involving theinitial brush application of thin coats of solvent-thinned polysulfide followed bythe application of a fillet of solventless polysulfide. It has been found that brushapplication of thinned sealant provides good adhesion to the substrate whereasthe fillet of solventless polysulfide provides good fuel sealing properties.

The polyester sealant (ii) was originally selected for use because of its excellenthigh temperature properties. However, its susceptibility to hydrolytic breakdownwas never examined and, as a result of this shortcoming, extensive fuel leaksoccurred in RAAF and USAF aircraft after a relatively short period of time. As aconsequence of the polyester breakdown, all RAAF FiI1 aircraft underwent anextensive deseal/reseal program between 1977 and 1982. To augment theresistance of the polysulfide sealant to penetration caused by expansion of tiehydrolysing polyester [1,2], a relatively rigid epoxy polyamide material wasapplied across all gaps as a supplementary barrier. Polysulfide was then appliedover the epoxy barrier coating to protect it from aviation turbine fuel and fueladditives.

FUEL AREA

GENERAL SEALANT CORROSION(F MS-1044) C

PROTECTION

COAT INGFAYING SURFACES

SEALING GROOVES

FILLET SEALANTFASTENER (FMS-1043)

INSTALLATION

STRUCTURAL VOIDS

AERODYNAMICSMOOTHER

NON-FUEL AREA

L.J

Figure 1: Original sealant system employed in Fl l1 fuselage fuel tanks.

From the time of the first deseal/reseal program until adoption of the presentfindings for the second deseal/reseal program, resealing of the F1ll fuel tanks hasinvolved:

(i) repainting of integral fuel tank surfaces where sealant is to be applied andcuring the paint for three days at ambient temperatures,

8

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(ii) application of a solution of a so-called 'titanate adhesion promoter' overthe paint, drying for 30 minutes, and removing adhesion promoterresidues with a solvent,

(iii) filling all gaps between mating surfaces with a thin continuous bead ofepoxy barrier coating and curing the epoxy for 24 hours,

(iv) reapplication of adhesion promoter, drying and removing residues (as in(ii)),

(v) application of two layers of solvent thinned polysulfide and two layers ofsolventless polysulfide sealant over the epoxy barrier coating and acrossadjacent painted surfaces to a width of approximately 3 cm.

This repair system has worked to a limited degree, however reports of fuel leakswere received by the mid-eighties, The severity of the problem graduallyincreased (Tables I & 2) to the extent that a decision was made to undertake asecond deseal/reseal program. In view of this decision, RAAF requested MRL toreview FIll tank sealing procedures [31 for possible improvements to long termdurability. The present report summarises the results of this investigation and ofchanges in RAAF procedures adopted for the second Fill deseal/reseal program.

2. Sealant Failures in RAAF Flll Aircraft

RAAF defect reports have generally indicated the number of leaks, the tanksinvolved, operational concerns, problems encountered and a discussion of thepossible causes of the leak 14, 5J. Difficulty in locating the cause of a fuel leak isfrequently experienced as the point where the fuel appears externally may besome distance from its source within the tank. This difficulty increases the timeinvolved and cost of repairs [6], e.g. a fuel leak from the aft glove area of the F2fuel tank just forward of the rotating glove above the L/H engine intake of FIllaircraft A8-132 required 608 manhours ($19,200) to be expended for location andrepair of the fuel leak.

The incidence of fuel leaks has gradually increased since the first deseal/resealto the extent that multiple leaks may be present in each aircraft [71, e.g.preservicing leak checks carried out on aircraft A8-109 revealed over 94 f el leakson the fuselage and wings including potential SOF (safety-of-flight) problems withleaks into enclosed space of the aircraft backbone channel. Detailed internal(visual) inspection of most fuselage fuel tanks revealed "areas of sealantreversion' , badly applied sealant, many attempted patch repairs, as well as areaswhere sealant had not been applied at all. The unit recommended that in view ofthe flight safety ramifications of some of the leaks found on this aircraft, a fuelleak survey of FIll fleet should be undertaken to assess the need for any

I The term reversion has commonly, but erroneously, been used to describe the sealant degradation

process. The cause of the degradation was hydrolysis which is different to reversion.

*" 9

acceleration of the deseal programme for the remaining wings and for the possiblecommencement of a programme of fuselage deseals.

RAAF Headquarters Logistic Command (HQLC) began requesting reports of allfuel leaks in FllI fuel tank by 1987. The accumulated records show that ahoperational aircraft suffered fuselage sealant failures between the period 1988-91(8]. Some of the failures involved poor workmanship such as inclusion of vwids orinsufficient sealant application, however many failures were observed wherematerials and procedures appeared u, have been followed correctly.

Table 1: FIll Fuselage Fuel Tank Summary - as at June 91

AIR•RAFP NO 1988 1989 1990 1991

AB-109 6 33 DS/RS 10AS-112 6 4 DS/RS IA8-113 8 8 DS/RS DS/RSA8-114 5 4 Pt DS/RS 2A8-125 NR 4 6 48A8-126 17 NR 15 DS/RSAS-127 3 5 16 DS/PSA.-129 NR 5 22 NILA8-130 3 9 10 NILA8-131 NR 8 DS/RS NILA8-132 NR NR 7 DS/RSA8-134 7 17 R5 NILA8-135 2 7 18 NILA8-138 4 3 R5 NILA8-140 3 8 30 NRA8-142 2 NR 5 NILA8-143 NR 2 R5 NILA8-144 NR NR 2 NILA8-145 NR NR 2 NILA8-146 NR NR 38 NILA8-147 NR 6 1 1AS-148 1 NR NR NIL

DS/RS = Undergoing deseal/reseal programPt DS/RS = Partly desealed/resealedNR = not recorded.R5 = aircraft service

10

L Jot ::

Table?: FIIl Fuselage Fuel Leak Summary - as at June 91

AIRCRAFT 1988 1989 1990 1991

A8-109T, Upper F2,L*8 Aircraft F2,L,2,3*

De/reseal S.R.1 P2,R-I underwent F2,R,2,3*2DEC76 S,L.1,4 FIL,12 part DS/RS T, Lower*iUSA SR.,3 TR,1-2 at 3AD/482 S,R,2-1

AI,R,4 Al/A,2'13 May 89-0ct 90 SL,2I1S.L,1-5 AI,L2'2S.R,1*3 DS/RS

schedl.ledl

A8-112S,R,1 F2,R,1,1-3 T, lower

De/reseal F2,R,13 AI,LI Work inSEP78 USA F2.L,12 Underwent this areaNOV90 USA DS/RS USA

A8-113ALKR1-4 AtR,1-4 R5 completed. OS/ES

De/reseal AI.L.1-4 AI,R.I-4 undergoing completed inOCT77 DS/RS JUN91USA at SMALC

A8-114FI,L.1.2 AI,R,2*2 A~rcraft had AI,L.2

De/reseal F1,R.2 Al,L.2*2 DS/ RS on Al O, S/ESNOV77 A2, S tanks NOV91USA at 3AD89/90

AB-125F2,R.-4 A1,R,4*3 F2,R,1'2

De/reseal F2,L.2 A2,1-1-3 S,L3*3,4*6MAR.80 Al/A2*13 SR.1*5,2*6

A-8-125SRY35,4*6A2,L.?,34XA2,L,4*2A2,R,1*2,3*3A2,RAý

A8-126EI,R,3A F2,L,1,2,3 UndergoingF2,RA.,2,3 AI,L,3 OS/US AMBF2,L,1,2 AI,R,2 by H de HS,R.2,3 A2,L,3Al,R,4 A2,R,1.3,4AI,Lz,44 SL,1,2,3,4A2,R,1 SR,3A2,L,1,4

Table 2 (Coutd): FIJI Fuselage Fuel Leak Summary - as at June 91

AIRCRAFT 1988 1989 1990 1991

A8-127F2 12,R-3 FI,L,4

De/reseal R,1,2,3 F2,L,1,3 F2,R,2"2MAR80 SR,1 F2,R,3-7

S,L,1 F2,L,S,L,2"3S,L,3

A8-129F2,R,3 F2,R'3 AI,R,2 DS/RSat

De/reseal F2,L,P12 A2,R.1 A2,R,2 SMA.CMAY79 SRI2 F2,L,3 JUN91

S,L,1-3 F2,R,2-2F2,R,3S,L,3,4,S,R,3,4

A8-130AI,R,1,2 F2,L,2A2,L,1,2 F2,R,1*3S,L I F2,R,2°3

AI,L,2-3AI,L,3"2

FWD, SIDE, BAY ............ e.g. F2, R, 3AFT, SIDE, BAY ............. e.g. A2, L, 2SADDLE, SIDE, BAY.....e.g. S, R, 1* -........................................... M d tip ly

During aircraft overhaul, personnel involved with deseal/reseal occasionallyobserved the appearance of small protrusions of reverted polyestcr sealantthrough the polysulfide sealant. The mechanism by which the degrading polyesterovercame the rigid epoxy barrier coating to reach the polysulfide is unclear butthe pressure of the reverting sealant [1,21 has presumably exceeded tb'

paint/epoxy bond strength thereby lifting the barrier coating from the paint. Thedegraded polyester then presumably migrated through this separation topenetrate the overlying sealant.

Discussions with RAAF personnel invo!ved in deseal/reseai of FllI aircraft

have provided some support for the suggestion that degrading sealant overcomesthe barrier coating by this mechanism. They described how long strips of sealant

which failed could be manually pulled from the seams, separating by adhesivedelamination of the barrier coating and the paint. While the adhesion between thepaint and the barrier coating is weak, the bond between the barrier coating and

the relatively soft sealant has generally remained intact during service so that thesealing material separating from the tank has been a strip of combined barrier

coating/sealant.

3. Experimental

3.1 Materials

3.1.1 Paint

The paint employed throughout this trial was polyurethane MIL-C-27725BType 11, Class B. DeSoto Inc., curing solution/base/solvent reducer 823-707/916-702/020-707. Although referred to as a polyurethane, the paint comprise- a longchain epoxy, cured with an isocyanate. The paint was mixed in the ratio of 4/1/4of curing solution/base/solvent reducer and let stand for 30 minutes prior tospray application. The panels were spray applied and cured for the specifiedperiods prior to application of the epoxy barrier coating.

3.1.2 Barrier Coatings

Of the 3M barrier coatings used in the Fill aircraft, two batches wete trialled. Thecurrent barrier coating, EC-3598 was compared to a two-year old sample of theEC-2216 which was formerly used. EC-3598 was a high viscosity version of EC-2216.

EC-2216, 3M Co., Lot No. 31M6R/93L6R (7parts by weight of A/5 parts byweight B);

EC-3598, 3M Co., Lot No. 96K4/4A5 (150 pts. A/100 pts B v/v); 100 parts base(B): 140 parts accelerator (A) Scotch-Weld.

Prior to application of the barrier coatings, EC-2216 or EC-3598, the paints weresolvent-cleaned with MIL-C-38736, a solvent mixture composed of aromaticnaphtha 50%, ethyl acetate 20%, methyl ethyl ketone 20% and isopropyl alcohol10% by volume. One group of paints was treated with the recommended adhesionpromoter, PR-148 (Products Research Corporation), a formulation understood tobe based on a 1% solution of an alkoxy titanate in the MIL cleaner. For the peeltest, the L.rrier coatings were applied with a Gardner knife at an averagethickness of 2 mm and cured for 24 hours at 25 °C prior to application of thesealant. For the lap shear tests, the ba,'rier coating was applied to a thickness of0.33 rm..

Alternative candidate barrier coatings examined were:

(a) Polyamide (PA); 139 parts Versamid 253, Henkel (Aust); 100 parts E9-1 10,Epirez Aust. Pty Ltd.

(b) Cycloaliphatic amine (CAA); 90 parts; Versamine C31, Henkel (Aust), 100parts E9-110, Epirez Aust. Pty Ltd.

(c) Polyurethane (PU); 20 parts Desmophen 1100; 80 parts Desmophen 550U;160 parts Desmodur L67, Bayer (Aust).

(d) Epoxy/polysulfide (EP/PS); 100 piis Capcure 3800, Henkel (Aust); 100parts E9-110; Epirez Aust. Pty Ltd.

13

,i

Alternative candidate coatings to the 3M materials were only examined by the lapshear test.

3.1.3 Sealants

Before application of the sealant, the epoxy barrier coating was lightly abraded,wiped with the MIL-C-38736 cleaner and treated with PR-148. The Class A sealantwas then applied. The Class B was mixed and applied over the Class A materialafter 6 hours. The sealants employed were:

PR-1750 A-2 MIL-S-83430A AMD 1,PR-1750 B-2 MIL-S-83430A AMD 1.

3.1.4 Aluminium Panels

All test panels were of 2024-T81 aluminium which had been deoxidised andpretreated with a chromate conversion coating prior to painting. The side to bepainted was cleaned with MIL-C-38736 and painted as per 3.1.1.

3.1.5 Adhesion Promoter

The adhesion promoter, PR-148, was freshly formulated from a concentratedsolution immediately prior to application of the barrier coating or sealant.Formulation of the adhesion promoter was to a 1% v/v solution in MIL-C-38736solvent. Adhesion promoter was then applied to the surface and allowed to dryfor 30 minutes after which time residual adhesion promoter was wiped from thesurface.

3.2 Examinations

3.2.1 Modified Peel Test

The modified peel (and lap shear) tests were carried out using an Instron Model1026 instrument with a cross-head speed of 50 mm/min and a full scale load of200 N.

The peel test was modified as follows. A piece of tape was applied across oneend of a painted panel to produce a non-adhering band of barrier coating/sealantfor the jaws of the Instron machine. The epoxy barrier coating was then applied tothe paint and cured for 24 hours (with or without adhesion promoter) prior toapplication of sealant. During application, wire mesh was applied into the sealant

to increase its cohesive strength. Prior to testing, the 2.5 mm wide non-adheringstrip of barrier coating/sealant and the panel with a narrow cut through theepoxy, were gripped and the peel strength measured (see Figure below). Providedthat the epoxy barrier coating was kept relatively thin (approximately 0.5 mmthick), the combined epoxy-sealant on the paint was sufficiently flexible tosuccessfully measure peel strength. This modified peel test provided a method toobserve whether (i) adhesive delamination would occur between the paint and the

14

barrier coating, (ii) adhesive delamination would occur between the barriercoating and the sealant, (iii) cohesive failure would occur within the sealant.

SEALANIBARRIER COATIN6 _ CUl

PANEL __

3.2.2 Modifd Lap Shear Test

Barrier coating was applied to appropriate areas of pairs of painted panels andeach pair was then bonded (by the drying barrier coatings) by the application of10 kg weight for 24 hours to produce an overlap of 12 mm. Some of the paintedpanels were treated with adhesion promoter prior to application of the barriercoatings. The panels were cut into five separate strips, 2.54 cm wide, andimmersed in fuel for appropriate periods of time. Lap shear tests were alsocarried out with paints which had an accelerated cure involving 24 hours at roomtemperature followed by 24 hours at 60 °C.

The panels were tested on the Instron using a crosshead speed of 0.5 ram/minand a full scale load of 10 kN. Each panel was also inspected to determine themode of failure in the test.

3.2.3 Fuel Immersion of Barrier Coatings

The barrier coatings were immersed at 250 C in (i) standard F35 (Avtur fuel) and(ii) F35 (fuel with the fuel system icing inhibitor (FSU) diethylene glycolmonomethyl ether at a concentration of 0.15% v/v). The effects of fuel immersionon the sealant system were monitored through percentage weight increase andpercentage volume swell (by ASTM 471).

4. Results

4.1 Techniques for Determining F1ll Sealant SystemPerformance

The initial difficulty in determining sealant performance was to devise a techniqueto measure the strength of adhesive or cohesive failure in a sealant systemconsisting of three layers viz, paint/epoxy barrier/sealant. (Adhesive failuresoccur by failure at the interface between two materials whereas cohesive failuresoccur when the bonds between materials are stronger than the cohesive strengthof the weakest component of the system.) A number of measurement techniques

is

were examined and modified peel and lap shear techniques devised (Sections3.2.1 and 3.2). The pad test was central to much of the work in that it provided areproducible method for comparing adhesion at the interfaces against thecohesive strength of the sealant which is the weakest material.

In practice, the modified peel test generally produced failures either by adhesiveseparation at the paint-epoxy barrier coating interface if adhesion was poor or bycohesive failure within the sealant if adhesion between paint/barrier coating wasgood. The lap shear test results were consistent with those of the peel test andprovided confirmation of the mode of failure. These techniques were used todetermine the effect of adhesion promoter, of barrier coating composition and ofcure schedules on the strength of the total sealing system.

4.2 Effect of Sealant Procedures on Adhesion and Cohesion ofSealant System

4.2.1 Effect of Paint Cure on Peel and Lap Shear Strength

Adhesion of the priming paint/3M barrier coating/sealant system increased withpaint cure time up to two weeks and resulted in increasing cohesive failure (anddecreasing adhesive failure) within the sealant system. Increased paint curetemperatures similarly improved adhesion as well as reducing the time requiredto achieve cohesive failure CTables 3, 4 Figure 2).

For shorter paint cure times, adhesive failures (by peel test) allowed the barriercoating/sealant to be manually peeled from the paint in the manner described byRAAF operators for in-service failures. Microscopic examination of the failedlower surface of the barrier coating (Plates Ia & Ib) showed it to have a surfacetextured in a cellular pattern characteristic of Bdnard Cells [9]. This effect, causedby retained solvent (see Discussion), effectively separates the paint and barriercoating except along the Bdnard Cell boundaries. It was found that relatively longpaint cure periods (6 days at 25 °C) were required to avoid Bdnard Cell formationand the adhesion problems associated with it. Similar adhesive failures wereobserved in lap shear tests when the paint had short cure times.

16A4

Lap Shear Strength (MN)

45

as.

ZYEMM 2C8G WHOMU MHINE

Figure2: Lap shear strength ofPR 1750/Barrier coating/Paint system vs Paint Age.

Lap Shear Strength (MN)

45.

PSAP

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4.2.2 Effect of Adhesion Promoter on Adhesion

Examination by both peel and lap shear tests showed that the adhesion of thebarrier coating to well cured paints (two years. two weeks, accelerated cure) wasgreatly reduced when adhesion promoter was applied to the paint. Paints treatedwith adhesion promoter generally displayed weak adhesive failure between thepaint and the barrier coating (Tables 3 & 4; Figure 3).

Systems in which paints had only short cure times generally failed by adhesiveseparation between the paint and the barrier coating whether or not they had beentreated with adhesion promoter.

4.2.3 Effect ofBarrier Coating Composition on Adhesion

Of the barrier coating compositions examined, the current system wasconsistently found to have the best adhesion to paint. Where the paint had beenfully cured (two years. two weeks, accelerated cure) the cycloaliphatic aminebarrier coating was also found to have good adhesion to the paint. Both thepolyamide and the polyurethane displayed reasonable adhesion to fully curedpaint although, surprsingly, the polyurethane barrier coating failed adhesivelybetween the sealant and itself. This was the only well cured system to fail in thismanner. The epoxy/polysulfide had poor adhesion to the paint (Tables 3 & 4;Figure 4).

Lap Shear Strength (kN)4J,

3

2

tj

PIS CAA PA Pu EMPs

PS = Present System PA = PolyamideCAA = Cycloaliphatic amine PU = PolyurethaneEP/PS = Epoxy Polysulfide

Figure 4: Lap shear strength vs. Barrier coating composition.

If 23

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With reduced paint cure times (7 hours, 24 hours, 48 hours), the adhesion of thecycloaliphatic amine barrier coating decreased significantly, giving, for instance,only fair adhesion on paint cured for 7 hours. The next strongest adhesionproduced after short paint cure time was the polyurethane barrier coating whichimproved with decreasing paint cure time. The polyamide system also exhibitedreasonable adhesion, however the epoxy/polysulfide barrier coating was againthe weakest of those tested.

4.2.4 Effect of Fuel Immersion Times on Swelling and Adhesion

The paints and sealants were not significantly swollen on immersion in Avtur(F35) fuel while the cycloaliphatic amine, polyurethane and polyamide barrier

coatings also exhibited minimal swelling. Significant swelling (as determined byweight increase) was observed, however, with the present barrier coating and the

epoxy/polysulfide barrier coatings, showing weight increases of 1.4% and 1.1%

respectively after three weeks (Table 5). When examined by volume swell in

Avtur, the increases in the present barrier coating after two, three and 35.5 weekswere 1.4%, 2.06% and 6.6% respectively. The changes in volume swell in F34RAAF fuel (avtur containing the antiicing inhibitor diethylene glycol methyl ether

(DGME) at 0.15%) were 1.7%, 2.5% and 7.1% respectively (Figure 5).

Table 5(A): Weight inc;-ease on immersion in Avtur

Barnier Coating Sealant Psent Weight before Weight after Weight afterImmersion One Week Two Weeks

Paint Only No 17.52 17.52 17.53Present System (PS) No 19.93 20.13 20.20

Present System (PS) Yes 46.09 46.28 46.27

Polyamide (PA) No 18.79 18.83 18.85Polyamide (PA) Yes 48.16 48.22 48.17

Polyurethane (PU) No 18.56 18.60 18.60

Polyurethane (PU) Yes 49.50 49.63 49-55Cycloaliphatic Amine (CAA) No 18.84 18.86 18.87

Cydoaliphatic Amine (CAA) Yes 42.00 42.10 42.03

Epoxy Polysulfide No 19.27 19.44 19.48

Epoxy Polysulfide Yes 39.61 39.72 39.68

Table 5(B): L'crcentage volume swreli of barrier coating in aviation fuel

Immersion Time (Weeks) Avlur/DGMiE Avtur

2 1.7 1.43 2.5 2.0

35ý5 7.2 6.6

24

70

60 A/7

50 //

S40

S30o

20 F : FAINT THINNER.IML-C-38734 CLEANER

1010 V

0 14 28 42 56 70 84 98 112 126 140

IMMERSION TIMR (HOURS)

%VOLUME SWELL OF BARRIER 3580

Figure 5: % Volume swell of Barrier 3580.

The swelling exhibited by barrier coatings in Avtur was eliminated or reducedwhen overcoated by the polysulfide sealant. Longer tests may be needed to gaugethis effect. An investigation of the volume swell of the barrier coating in the paintthinner (020-707) used for the Integral Fuel Tank Coating showed it to have agreater, albeit slower, effect on swelling than did the MIL-C-38736 cleaningsolvent (Figure 3).

No difference in the adhesion of the complete sealant system was observed afterone and three weeks immersion in F35 fuel.

4.2.5 Effect of Aging of the Barrier Coating

When aged 3M EC2216 barrier coatings were examined, adhesion was reducedcompared with adhesion of fresh samples. Microscopy of the aged coatingsshowed the presence of voids (Plates 2a). Sectioning of the sample for microscopy

25 t

SI'•

p .

led to separation of the sealant and barrier coating (Plate 2b). Analysis of the

water content of the barrier coatings by the Dean and Stark method showed that

the aged sample had higher water content which may have contributed to the

poor adhesion. In view of these results, aged barrier coatings were discarded-

Water contents were found to be:

Batch 1 (aged sample - 2 years) Batch 2 (new sample)

EC 2216A 1.2% EC 3598A 02%EC 2216B 0.2% EC 3598B 0.3%/o

5. Discussion

A number of author- have investigated sealant failures characterised by

separation at the paint/sealant interface. Usmani et al. [101 reported that thesealant/coating combination works well initially and there is apparently adequatc

adhesion of the polysulphide rubber to the polyurethane coating. Upon extended

aircraft usage, however, leaks do develop which cannot be repaired by removingthe old polysulfide rubber and reapplying fresh sealant because of insufficientadhesion. Usmani 1101 also found that heat treatment of polyurethane paint at121

0C produced a surface to which the polysulfide would not stick and that the

adhesion of the polysulfide sealants slowly deteriorated with age of the paint [11-

13]. They correlated the loss of adhesion with the loss of unreacted isocyanate

groups on the polyurethane surface, disappearance of these groups beingaccelerated by increased temperature and humidity. In a similar study on the

effect of aging of epoxy priming paints on sealant adhesion, Wilford and Day 1141found that adhesion of one of the epoxy paints decreased significantly with aging

of the primer but three other 'tolerant' primers were unaffected. The cause of this

difference was not determined.Adhesive failures associated with the overpainting of aged epoxy paints are

well known in the paint industry even at drying times of less than 24 hours. Prior

to the present investigation, excessive aging of the primer was considered a

possible cause for the sealant failures plaguing the RAAF Fll aircraft. However,the finding of the opposite phenomenon, i.e. good adhesion of the barrier coating

to aged paints but poor adhesion to freshly cured paints stimulated further work

to determine the cause of the adhesive failures in the Fi 11 tanks.

26

Plate 2a: Void formation in aged barrier coating.

Plate 2b: Separation of Paint and Epoxy Barrier Coating (Aged).

27

The long paint cure times needed to achieve good adhesion (Table 6) weresurprising given the film thickness (ca 20-25 urn). The slow evaporation ofcyclohexanone from the paint solvent has been reported to require an extensiveperiod of time [15]. Furthermore, isocyanates can also complex with ketones toform ketimines which will further slow solvent evaporation. Where the presenceof ketimines is suspected, the application of heat is often used to assist paint cureby driving off the ketone produced from ketimine breakdown.

Table 6: Results of Peel Strength Adhesion

Paint Cure Times (hours)

Temp C IS 17 19 20 24 25 35 40 44 48 50 54 72 96 120 144

25C F F F F F F F F F F F F F F F/P P

37C F F F F F F F F/P P P p P P P P P

40C F F F F F F/P P p P P P P P P P P

50C F F F F F/P P P P P P P p p P p P

60C F/P P p P P P P P P P P P P P P P

F= Fail P = Pass

Adhesive separation = failure (i.e. when adhesive strength is less than 20 N/mm at a cross head speedo50 mrm/amin).

Cohesive separation = pass (i.e. when adhesive strength is greater than 20 N/mm).

The cause of the poor adhesion in the sealing system became apparent with theobservation of Bdnard Cell formation under the epoxy barrier coating. BMnardCells are normally formed on the surface of a paint film by currents set up as aresult of solvent circulation. When the solvent volatilises at the surface of a wetfilm, solvent in the body of the film migrates to the surface to replenish thesurface supply. To facilitate solvent escape, small pores form which terminate atthe surface of the film. These pores commonly result in the formation of geometricpatterns observable on the surface of the film as the paint being emitted fromthese eruptions spreads away from the pore to the outside of this crater [9].Boundaries are formed between adjacent pores resulting in the typical cellularstructure seen in Plate la and lb. However, in the present case, a solvent mixtureis being released to the surface of a paint film overcoated with a thick, epoxypolyamide barrier coating. Curing epoxy polyamides contain long fatty acylcomponents which are susceptible to solvent effects such as the formation ofB~uard Cells. Somewhat unusual, however, is the fact that the B6nard Cells areformed on the mating or mirror surface to the paint from which the solvent isleaving.

In an effort to overcome the poor performance of fuel tank repairs, GeneralDynamics 1161 conducted a study on improving adhesion between sealant andpaint by the use of adhesion promoters. This study, by Carroll and Pritchard 1161found that the adhesion of paint to sealant was improved by some adhesionpromoters but not others. For systems where the adhesion promoter wascompletely ineffective, abrasion was required to provide a mechanical key forimproved adhesion. Usmani et al 1101 similarly found increased bond strength

28

with an adhesion promoter (an alkyl phosphatotitanate) although theirobservation that a solvent wipe removed the adhesion promoter from the paint issurprising in that it precludes chemical bonding which is the basis of action of theadhesion promoters.

Application of an epoxy barrier coating between the paint and the sealant forFllI aircraft was reported in a study by General Dynamics [171. The barriercoating originally chosen was 3M's EC-2216, however this was later changed inservice to 3M's EC-3598. The adoption of a barrier coating appears courageous inview of a number of unresolved problems described in the report. These problemsincluded loss of adhesion of the sealant to the epoxy barrier coating with time andtemperature, loss of flexibility on exposure to low temperatures (cracks in thebarrier coating), as well as loss in hardness, loss in flexibility and increasedswelling on fuel immersion. The authors obviously placed great faith in thepolysulfide overlay to alleviate many of these problems suggesting that for actualfuel tank applications, however, the epoxy will be covered by a thick fillet ofpolysulfide sealant which should protect the epoxy from the fuel 1171.

The results of the fuel immersion tests in the present investigation support theGeneral Dynamics contention that the use of polysulfide reduces swelling in fuel.However, care must be taken with results from short term trials with sealants. Acase in point is that at one time, USAF used sealants in fuel tanks without anunderlying priming paint. They did this on the basis of laboratory evaluationsshowing that the sealant alone would protect the tanks from corrosion. Theybased this on short term exposure results [181. In service, however, the onset ofcorrosion occurred several months later.

Examination of the effect of barrier coating composition on fuel tank sealantperformance supports the barrier coating selected by General Dynamics.Although the present barrier coating was swollen by fuel, it consistently exhibitedthe best bond strength to priming paint of all candidate materials. Evaluation of apolyurethane as a barrier coating, showed it to have moderate adhesion to thepriming paint but poor adhesion to the sealant. On the other hand, it exhibited thebest adhesion to freshly cured paint, presumably by interaction of the isocyanateswith amines and hydroxyls in the barrier coating. The epoxy polysulfideconsistently exhibited poor adhesion.

The adhesion of sealant systems using barrier coating in combination with theadhesion promoter, as used in the FIll fuselage tanks, was never examined; it istherefore not surprising that adhesion failures occurred in service. The presentreport has shown that this combination results in poor adhesion and thatremoving the adhesion promoter significantly improves the durability andstrength of the sealant system. Likewise, paint cure has a significant affect on theintegrity of the barrier coating and its adhesion to the priming paint. Bymodifying these two procedures, a system for sealing Fill fuselage fuel tanks isachieved which has adhesion greater than the cohesive strýngth of the polysulfide.

6. Operation of the Second DeseallResealProgram

Although the changed deseal/reseal procedures were largely finalised prior todesealing the first FIll aircraft, the onset of the resealing program in the aircraftrequired that the range of conditions under which procedures could be employed

29

ii .i;,..~

be gradually expanded. This expansion was required to meet material curingrequirements under the environmental conditions prevailing at RAAF Amberley.The evolution in procedures 119-201 is contained in MRL and RAAF minutes andis instructive for operational considerations.

When the paint cure requirements were originally investigated prior to thedeseal/reseal programme commencing, it became apparent that good adhesionbetween the paint and the barrier coating required paint cure times considerablylonger than those requiring the paint to meet the solvent resistance specificationwhich existed up to that time. It also became apparent that a minimumtemperature requirement was needed for adequate paint cure.

When resealing of the first two Fill aircraft commenced at Amberley,temperature control equipment was unavailable at the overhaul facility. As aresult, the contractor undertook resealing of these two aircraft using an ambienttemperature cure regime with relatively long cure periods. However, thetemperature at the facility fell well below the minimum temperature requirementat night. As such, it was impossible to determine the elapsed total cure time abovethe minimum required temperature.

As a result of installation of heating capacity at the overhaul facility, aconsiderable range of temperature regimes became available for the reseal ofFIll's after aircraft No 2. MRL was therefore tasked by RAAF with determiningwhether the IFTC overcoat curing temperature range could be expanded from aslow as 20 oC up to 50 oC and on whether the cure could be accelerated under hightemperature up to the point of initial fuel/MEK resistance followed by reducingthe temperature to final cure. RAAF also requested that MRL examine thepossibility of producing a table of cure times to achieve final cure. [21].

Following further evaluation, the conditions of cure were then expanded toaccelerate the cure of IFTC by the use of high temperatures with a restriction forletting the paint flash dry for 30 minutes after application. It was found that it waspossible to accelerate the IFTC cure under high temperature and subsequentlyreduce temperature, or conversely gradually increase temperatures during cure.However, it was found that it was not permissible to partially cure the paint athigh temperature, apply the barrier coating and allow it to cure. This proceduredid not develop maximum adhesion between the barrier coating and the paint(221.

In view of the quick overhaul times achievable using elevated temperatures,RAAF further requested that paint cure schedules up to 60 oC be considered [23].Following further work, time schedules were compiled at temperatures up to60 0C (Table 6) which met the paint cure requirements [241. Because of daytimetemperature variations, this tabulation was subsequently converted to a graph ofso-called 'heat units' (Figure 6) achievable at various temperature-time schedules,a heat unit being defined as the percentage cure gained per hour at a specifictemperature, e.g. cure is achievable in 144 hours at 25 °C. Therefore, heat unitsper hour at 25 oC = 1/144 x 100 = 0.69% [25]. These units are additive so that ifthe temperature cycle of an aircraft is known, then the ddditional time required toachieve acceptable cure can be calculated. Once full cure is achieved (100%), theintegral fuel tank paint could be overcoated with barrier coating.

30

2"S

7.0 -

4.0

5.S

S.0

4.5 :'

0/

4.0

3.Sz

3.0

Ls

1.0

SO /

20

TEMPERATURE C

MINIMUM CURE TIMES

FOR INTEGRAL FUEL TANK COATING (PAINT)

Figure&6 Minimum cure times for integral fuel tank coating (paint).

7. Conclusions

From this study, the following conclusions have been drawn:

(i) Sealant performance in the fuselage fuel tanks of FillI aircraft was criticallydependent on the adhesion of the barrier coating and the sealant applied over it,to the Priming paint used to protect the tank surfaces. Two factors were found tocritically affect the adhesion of the barrier coating and sealant to this primingpaint. These factors were (a) the requirement for adequate cure of the primingpaint prior to application of the barrier coating and sealant, and (b) the use of anadhesion promoter between the paint and the barrier coating.

31

Ii j%

(a) An increased degree of cure of the priming paint is needed beyond theformer requirement of resistance by the paint to methyl ethyl ketone. Thepaint contains the low volatility solvent cyclohexanone which requiresextended cure. Failure to carry out proper cure results in solvent attack onthe underside of the barrier coating and poor adhesion between the paintand the barrier coating.

(b) The use of the recommended adhesion promoter between the paint andbarrier coating was found to lead to greatly reduced performance by thefuel tank sealant system. The adhesion promoter was found to reduce theadhesive strength between the paint and barrier coating.

(ii) The present barrier coating is easily the best performed of those tested.

From the above it is recommended that for maximum adhesion of the barriercoating system, the present barrier coating should be retained and applied overwell cured priming paint. The painted surface should be solvent cleaned, howeverthe titanate based adhesion promoter should not be employed.

8. References

1. Paul, D.B. & Hanhela, P.J. Interactions Between Flll Fuselage Fuel TankSealants. Pt 1. Characterisation of Polyester Sealants and their HydrolyticDegradation Products. MRL-R-657

2. Paul, D.B. & Hanheta, P.J. Interactions Between F111 Fuselage Fuel TankSealants. Pt. 2. Variation in Performance Properties of Polysulfides afterContact with Polyester Degradation. MRL-R-658.

3. RAAF Report AIRI /404/A8/341 Pt 6 (61) of 1 May 1990. T. Roediger.

4. RAAF Signal, 3AD/80/90 523/OIC AMSQN of 20 June 1990.

5. RAAF Signal, 6 SQN/40/89, 187/SENGO of 18 May 1989.

6. RAAF Signal, 3AD/32/89, 337/OIC AMF of 20 March 1989.

7. RAAF Signal 482S/2506/36/Tech Pt 7 (44) GPCAPT K.R. Webber of29 Jul 91.

AIRI File Ref AIR1/4110/A8/4600 DOI.

Q Patton, T.C. Paint Flow and Pigment Dispersion. 2nd Ed. John Wiley & Sons.

(1978), 593-6.

10. Usmani, A.M., Chartoff, R.P., Warner, W.M., Butler, J.M., Salyer, 1.0. &Miller, D.E.I. Interfacial Considerations in Polysulfide Bonding. RubberChemistry and Technology, (1981), 54,1081.

32

• ' IL •. ,

i ' '

11. Usmani, A.M. Research on Polysulfide and Polyurethane Coating Interface, Org.Coat. Appl. Polym. Sci. Proc.. (1982), 47, 19-22.

12. Usmani, A.M. Application of Infrared Spectroscopy in Curing Kinetics ofPolyurethane Coatings. J. Coatings Tech. (1984), 56, No 716, 99-103.

13. Usmani, A.M. Chemistry and Technology of Polysulfide Sealants. Polym. Plast.Technol. Eng., (1982), 19(2), 165-199.

14. Wilford, S.P & Day, 1. The Effect of Primer Age on Adhesion of PolysulfideSealant. Report for Materials and Structures Department, Royal AerospaceEstablishment, Farnborough Hants, UK (1988).

15. Athanosopoulos, C.G. DeSoto Aerospace Coatings Inc. PersonalCommunication, 1992.

16. Carrol, M.T. & Pritchard, D.J. Adhesion Promoter for Fuel Tank Sealant TestProgram. General Dynamics Report FZM-12-13731 July 1974.

17. Carrol, M.T. & Pritchard, D.J. EC-2216/XA-3517 Epoxy Barrier Low TemperatureFlexibility and Heat Shrinkage Tests. General Dynamics Report FGM-5892 May1976.

18. Mr L. Riley. Sacramento Air Logistics Command (SMALC) ChemicalLaboratory. Personal Communication, 1988.

19. MRL Minute 490/202/0015. Investigation of the Cure Times for Integral FuelTank Coating under Different Environmental Conditions. L. Wake of August1990.

20. MRL Minute 490/202/0015. Cure Requirements for DeSoto Integral Fuel TankCoating. L. Wake of 25 June 1991.

21. RAAF Minute AIRI/4080/A8/341 Pt 11 (21). Additional IFTC CureInformation. SQNLDR P. McLennan of 20 Dec. 1991.

22. MRL Minute 490/202/0015. Additional IFTC Cure Information. L. Wake ofJanuary 1992.

23. Telecon L.Wake/SQNLDR P. McLennan of April 1993.

24. MRL Minute 490/202/0015. Additional Cure Information on Integral Fuel Tank

Coating (IFTC). R.H.E. Huang of 9 June 1993.

25. RAAF Minute AIR/4080/A8/341 pt. All (45). Integral Fuel Tank Coating -Cure Requirements. SQNLDR P. McLennan of 23 June 1993.

33

/~

SECURITY CLASSIFICATION OF THIS PAGE UNCLASSIFIED

REPORT NO. AR NO. REPORT SECURITY CLASSIFICATIONMRL-TR-93-64 AR-008-616 Unclassified

TITLE

Improved Resealing Procedures for the Second Deseal/Reseal Program in RAAF FllI Aircraft Fuel Tanks

AUTHOR(S) CORPORATE AUTHORS. Batten, R.H.E. Huang and L.V. Wake DSTO Materials Research Laboratory

PO Box 50Ascot Vale Victoria 3032

REPORT DATE TASK NO. SPONSORJanuary, 1994 AIR 91/141 RAAF

FILE NO. REFERENCES PAGES

G6/4/8-4696 25 34

CIASSIFICATION/UMITATION REVIEW DATE CLASSIFICATION/RELEASE AUTHORITY

Chief, Ship Structures and Materials Division

SECONDARY DISTRIBUTION

Approved for public release

ANNOUNCEMENT

Announcement of this report is unlimited

KEYWORDS

Sealants Paint cure Paint/epoxy bondCuring Bonding Barrier coatings

ABSTRACT

A high temperature-resistant polyester sealant was originally employed to seal faying surface grooves in Fl IIfuselage fuel tanks as well as structural voids and aerodynamic surfaces. Chemical hydrolysis of this materialwas detected in March 1974 and led to its replacement by a polysulfide sealant in the late 70's and early 80's.Where possible, the polyester was removed, however much of it remained inaccessibly located between theoverlapping panels. It was found that the continued hydrolysis of this residual polyester generated sufficientpressure to penetrate the polysulfide sealant resulting in fuel leaks as well as appearing externally in trails alongthe skin of the aircraft. To protect the polysulfide from penetration, a mechanical barrier was applied in the formof a layer of epoxy polyamide between the polysulfide and the polyester. This system was successful for someyears, however the occurrence of fuel leaks had become unacceptable by 1987. All Fill aircraft sufferednumerous fuel leaks over the period 1988-91 including a number of safety-of-flight (SOF) incidents. A detailedinvestigation was therefore requested to identify the cause(s) of sealant failures. This investigation found that thesealant and the underlying epoxy barrier coating could be manually peeled from the painted tank surface whenprepared by the recommended methods and that two procedures were responsible for the poor adhesion. Oneof the procedures was found to lead to incomplete cure of the underlying priming paint resulting in solventattack of the barrier coating. The second procedure, involving the use of a so-called "titanate adhesionpromoter", significantly reduced peel strength of the barrier coating to the paint. Modification of these two stepsresulted in adhesive bond strengths greater than the cohesive strength of the polysulfide sealant. These changeswere adopted for the second deseal/reseal program of the Flll aircraft fleet. Following teething problems withthe first aircraft, the procedures were improved and the FI I fleet remains leak-free after two years.

SECURITY CLASSIFICATION OF THIS PAGE

UNCLASSIFIED

" WA="

Improved Resealing Procedures for the Second Deseal/ResealProgram in RAAF Fill Aircraft Fuel Tanks

S. Batten, R.H.E. Huang and L.V. Wake

(MRL-TR-93-64)

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